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How do you limit output?
Posted: Tue Nov 02, 2010 11:54 am
by Frederic Gelinas
I read everywhere that you should limit your amp output in order to preserve your cab's health. It makes sense to me. What do I need and how would I setup if I want to limit the signal going to the cab? Is the procedure the same for tops and subs?
Re: How do you limit output?
Posted: Tue Nov 02, 2010 11:56 am
by Zack Brock
You obtain a brickwall limiter which is a piece of rack mount equipment that allows you to limit the input voltage to the amplifier. Or you buy an amp like a Crown XTI that contains a built in DSP which includes a limiter, or a dbx DriveRack or Behringer Ultradrive or any similar multi-function unit that contains a
brickwall (not just a hard knee compressor) limiter that you can configure.
Here is a good step-by-step on how to set your limiter settings, not sure I remember where I got this from but it was from another user here on the forums in an old thread:
Limiter Settings & dBu Calculations
--
You can calculate the maximum power available to the drivers and set
your limiters accordingly.
First you need to convert the power rating of your driver to Volts
rather than watts.
Lets assume you have a driver rated for 600 watts at 8 ohms nominal.
The formula would be....
Sqrt(watts*ohms) so sqrt(600*8) = 69.28 volts
Next you need to convert the volts to dBu using the formula 20 Log(volts/.775)
so 20 Log(69.28/.775) = 39.03 dBu
Now lets assume you have an amplifier capable of 800 watts at 8 ohms.
We now need to figure out mow much gain the amplifier has using the
first formula.
sqrt(800*8) = 80 volts. then we convert that to dBu also.
20 Log (80/.775) = 40.28 dBu
Now to find the limiter setting you subtract the amplifier gain from
the dBu required from the driver.
39.03-40.28 = -1.25 dBu
So you would set your limiter for 1.25dBu below full output of your amplifier.
So if your amplifier has full output at +4dBu then you would set your
limiter for +2.75dBu.
Now If your amplifier has a constant gain such as 26 dB or 32 dB gain
then you would subtract the gain of the amp from the driver dBu
So for 26 dB gain the math would be
39.03-26 = 13.03 dBu for the limiter
Or for 32 dBu then it would be
39.03-32 = 7.03 dBu for the limiter.
All of these answers would give you ~600 watts to the driver depending
on your particular amps settings. There you go, now your are a little
safer.
Re: How do you limit output?
Posted: Tue Nov 02, 2010 12:27 pm
by schadstr
hey guys, zack these formulas shuld also help me with setting up the limiter on my drpa+ also?
Re: How do you limit output?
Posted: Tue Nov 02, 2010 12:31 pm
by Zack Brock
schadstr wrote:hey guys, zack these formulas shuld also help me with setting up the limiter on my drpa+ also?
Yes but I don't think the DRPA+ is a brickwall limiter (the original DRPA was not, not sure about the +)
Re: How do you limit output?
Posted: Tue Nov 02, 2010 1:03 pm
by Ron K
I just keep the level under control by listening for cone breakup, coil knocking or any distortion that doesn't sound right. Using proper gain structure and amp matching the loads doesn't hurt any either.
It helps when you know where the real limits are in your rig and then you dont exceed them.If you cant do proper gain structure or amp to load matching then you should use a limiter to stay on the safe side.
Edit: Limiters in the Crown XTIs are not peak limiters but RMS feedback limiters. They are "not" brickwall limiters and not peak stop limiters!
Re: How do you limit output?
Posted: Tue Nov 02, 2010 2:58 pm
by Zack Brock
Good points Ron, except you can't hear cone breakup in a folded horn subwoofer
Re: How do you limit output?
Posted: Tue Nov 02, 2010 6:28 pm
by Greg Plouvier
Zack Brock wrote:Good points Ron, except you can't hear cone breakup in a folded horn subwoofer
Actually you can but a few seconds later magic smoke is released.
Re: How do you limit output?
Posted: Wed Dec 29, 2010 11:50 am
by el_ingeniero
Just remember to use the displacement limited voltage (volts required to send the driver cone to Xmax), if that is less than the voltage for the power rating.
Re: How do you limit output?
Posted: Wed Dec 29, 2010 10:48 pm
by Gregory East
That's the limit voltage that is specified in the plans for the recommended drivers for all the bass horns. It is frequently a lot less than the voltage you would put in to produce rated driver power in a radiator cab.